N-band Observations of He 2-10: Unveiling the Dusty Engine of a Starburst Galaxy

Abstract

To better understand the early stages of massive star cluster evolution we have obtained J,H,K', and N (10.8 micron) images of the nuclear region of the starburst galaxy He 2-10. The N-band images were obtained with the Gemini North telescope and reveal four of the five enshrouded clusters, or "ultradense HII regions" (UDHIIs), recently discovered in radio maps. None of these sources appears in either the optical HST images or the near-infrared images. They comprise about 60% of the total N-band flux from He 2-10 and, we suspect, a similar fraction of the total far infrared flux measured by IRAS. The inferred spectra of the UDHIIs are very similar to those of Galactic ultracompact HII regions and we have modeled the UDHIIs under the assumption that they are "scaled-up" versions of these objects. The bolometric luminosity of the brightest UDHII is estimated to be ~2x10^9 L_sun, and the total mass of the dust and gas is ~10^7 M_sun. The mass of the enshrouded stellar cluster must be > 2.5x10^6 M_sun and the age must be < 4.8x10^6 yr. All of the stellar clusters within the UDHIIs must have ages less than about 5x10^6 yr and masses greater than about 5x10^5 M_sun. The logarithmic ratios of the radio to far infrared flux densities for the individual UDHIIs, and He 2-10 as a whole, are significantly larger than the average for normal galaxies, but comparable to those for ultraluminous infrared galaxies. Large ratios for some starburst galaxies may indicate that a significant fraction of the far infrared flux arises from UDHIIs. If all of the far infrared flux from He 2-10 and other starbursts is produced by heavily obscured regions, the observed correlation between UV continuum slope and infrared-to-ultraviolet flux ratio in starbursts cannot be due primarily to UV reprocessing by dust in a foreground screen.